1. Field of the Invention
This invention relates to a coating method and apparatus which is suited for forming magnetic coating on magnetic recording mediums.
2. Description of the Prior Art
In the conventional equipment for producing magnetic recording medium such as magnetic tape, a substrate or tape base 1, as shown in FIG. 1, is fed from a supply roll 10 and is applied with magnetic coating by the known coating applier of the extrusion type. The coated tape is then sent into a dryer 12 where it is dried by hot air 14 blown from nozzles 13. Next, the tape 1 coated with magnetic layer is calendered by known calender rolls 15 and, after coating it with a black coat if necessary, is wound on a take-up roll 16.
In this case the tape 1 is continuously made to run while being supported by a backup roll 18. A coating roll 17 is pressed against the backup roll 18. The coating liquid delivered from an extrusion type coating applier 11 onto the coating roll 17 is controlled to the desired thickness by a metering roll 19 and transferred onto the tape 1 running on the backup roll 18. This technique of magnetic coating is known as the reverse roll method disclosed in Japanese Patent Laid-Open Nos. 84242/1977 and 17661/1981.
FIG. 2 shows the detail of the magnetic liquid applying section using the reverse roll method. More coating liquid 2 than is needed is applied to the coating roll 17 from the liquid applier 11. The metering roll 19 is so located that it contacts the coating roll 17 at a point before the liquid on the coating roll reaches the backup roll 18. The metering roll 19 scrapes excess liquid off the coating roll 17 to control the amount of liquid to be transferred to the tape. The excess liquid 2 is collected in a container 3 from which it is circulated by a pump 4 to a reservoir 5 and further fed to the applier 11 by a pump 6. This liquid circulating system has disadvantages. For example, some foreign matter may get into the liquid while the liquid is staying in the container 3 causing abnormal liquid application; the composition and viscosity of the coating liquid may change while it is being circulated; also the particles may coagulate during circulation. These result in variation in the thickness and characteristic of the magnetic layer formed on the tape, deteriorating the quality and stability of the product.
Another known technique of magnetic coating is a gravure coating. This process is performed in a system which is exposed to the open air, so the coating liquid evaporates into the air changing the liquid composition and deteriorating the quality of the product. Moreover, the gravure pattern of the coating roll is subject to wear during operation, which results in a change in the coating thickness. This requires frequent replacement of the coating roll, reducing production efficiency.
In the extrusion coating method disclosed by the Japanese Patent Publicaton No. 10110/1981, the coating liquid 2 is supplied in volume several times more than necessary, as shown in FIG. 3, from the reservoir 22 to a chamber 23 of a liquid applier 31 by a pump 21. The liquid 2 fed into the chamber 23 is further supplied through a slot 25 to a nozzle 26 at the end of the slot where the liquid is controlled to the desired amount by a doctor edge 27 which is mounted immediately after the nozzle 26 with respect to the tape running direction. The regulated amount of coating liquid is applied onto the tape 1 running in the direction of the arrow. The thickness of the coating is about one-half of the gap between the doctor edge 27 and the tape 1. The excess liquid 2 flows down, as shown at 24, by gravity through a larger gap between the tape 1 and the edge 29 which is mounted on the tape-incoming side of the nozzle 26. The excess liquid 2 thus collected in the reservoir 28 is then supplied by a pump 30 to the reservoir 22. However, since the excess is recovered to a reservoir 28 and then to the tank 22, the system shown in FIG. 3 has similar drawbacks to those of FIG. 2 such as change in liquid composition, coagulation, and viscosity variation.
To remove these drawbacks, it has been proposed that in the extrusion coating system, only the exact amount of liquid necessary to form a specified thickness of coating on the tape is extruded (that is, in FIG. 3, the liquid amount is reduced to eliminate the excess). In this case, however, fine longitudinal streaks like the wood grain of lauan are formed on the surface of the coated layer, appearing from near the nozzle of the liquid applier.
The extrusion coating system published in the Japanese Patent Laid-Open No. 19060/1982 has been devised to remove the longitudinal streaks on the surface of the coated layer. According to this, as shown in FIG. 4, the coating liquid 2 is introduced from a supply nozzle 32 into the liquid chamber 23 in the liquid applier while at the same time a part of the liquid 2 is discharged from a discharge nozzle 33 at the other end of the applier. The amount of liquid supplied to the chamber 23 is greater than that coated on the tape and the excess liquid is discharged from the nozzle 33. So it is considered that the streaks will not result. Although there is no description as to the process of the liquid after being discharged from the nozzle 33, the excess liquid is considered to be circulated and recovered to the reservoir or discarded. If the liquid is circulated for reuse, the aforementioned drawback is not overcome at all. And if it is thrown out, the loss of liquid will inevitably increase cost.